1. Field of the Invention
The invention relates to a process for mixing chemicals into a pulp suspension, preferably in the medium-consistency range, where a rotor is mounted inside a mixing chamber.
2. Description of the Related Art
In addition to the static mixers and so-called high-shear mixers, mixers are also known which make use of the so-called fluidization principle. Such mixers are known, for example, from EP 0 578 284, U.S. Pat. No. 5,279,709 and WO 93/17782. In these mixers, the high-speed rotation of the agitators exerts high shearing stress on the pulp, which then dissolves the pulp mesh and gives the suspension the physical properties of water. Here, a high-speed rotor is mounted in a suitable casing. Due to the relatively large gap between the rotor and the housing, the drive power is much lower compared with high-shear mixers and the individual fibers in the suspension are not destroyed (shortened). Since the dimensions of the housing are small, relatively high throughput rates are achieved and the pulp retention times in the mixer are relatively short as a result. Thus, only brief periods are available for the fluidization process. Particularly when gaseous chemicals are mixed into the pulp, the liquid and the gas separate because of the difference in density. When this happens, the gas moves towards the center of the rotor and is removed there from the mixture of gas and liquid. This effect is exploited by pumps used to convey pulps, particularly in the medium-consistency range, in order to be able to extract the unwanted air at the center of the rotor. Furthermore, mixers are known from U.S. Pat. No. 3,314,660 which can mix different pulps well in one tank. In this case, mixing takes place in a batch process in which the pulps are fed into the tank and then mixed. What is required of a good mixer to mix gaseous chemicals continuously into a pulp suspension is, however, even distribution of the fine bubbles of gas in the pulp. For the reason mentioned above, the mixers known to date do not fulfill the necessary requirements pertaining to even distribution of the chemicals, e.g. ozone, mixed into the pulp.